Distance vent stack to chimney

Is there a minismium distnace from the vent stack to a chimney. I guess worst case is that methane would travel down the chimney and explode when you light a fire. OK I said worst case. Is a chimney considered an "opening"?

I am thinking that sewer gas has to be lighter than air to go up and out of the pipe. It would have to be heavier than air to go down a chimney. So, I should be OK? What will the inspector think?

906.2 Each vent shall terminate not less than ten (10) feet (3048 mm) from, or at least three (3) feet (914 mm) above, any openable window, door, opening, air intake, or vent shaft, nor less than three (3) feet (914 mm) in every direction from any lot lineâ€“alley and street excepted.

I don't think that sewer gas rises out of the vent pipe like you think. The vent pipe is there to provide an equalizer to the rapid pressure changes that occur in your plumbing when you drain a sink, tub or run your dishwasher or washing machine.

Without a vent pipe, draining your sink could cause a nearby tub (or other fixture) to start filling with water through the drain. It can also cause your p-traps to get sucked dry leaving sewer gas free to enter your home through your drain.

I'm no plumber so I could be wrong, but that is my understanding of the purpose of a vent pipe.

I have never seen, and the stories I have heard may be apochryphal, of sewer gas exploding in a sewer, much less in a fireplace. The only thing that might happen is that the odor could be drawn into the house down the chimney if you got a downdraft or negative pressure from a large house fan.

My friend has an old house in Gloucester Ma. where the vent for the 1st floor kitchen sink (drum trap) runs out and up the side of the house terminating just below the eaves, that is obviously no good.
He's doing the kitchen over, 2 sinks and a washer, there is a pantry off the kitchen that juts out about 6' and that's where the washer will be.
I suggested venting all 3 fixtures through the pantry roof, only problem is there is a window within 4' horizontally from the pantry roof, if he can't do that he'd have to open the walls all the way up to the attic.
Do you think the inspector would allow the vent 4' away?
I believe the code in Ma. say that you need to terminate 2' above a window or 10' horizontally.